I see that he has 3 trim pots and I'm guessing that's to tune the input from a wavelength specific photoresistor of some sort. maybe you could use celophane covered photoresistors....never really heard of it but.....

if you have a look here: http://www.lynxmotion.com/images/jpg/color.jpg you will get the idea.it is illuminating the m&m with different colours of light.switch on the different colours of led, one at a time anda red m&m will reflect back more red light,a blue one will reflect back more blue light,etc.

ive been thinkin of making a color sensor tutorial for awhile now . . . and not just for visible light . . .

what dunk said is the cheapest/easiest way . . .

the taos sensors are for only a single color, so you would need multiple of those designed for different colors . . . those are higher precision, meaning they work great for telling the difference between shades of colors.